DE69117989T2 - Obtaining pure nitrogen from air - Google Patents

Abstract

A process for removing the impurities carbon monoxide, carbon dioxide, water vapour and, optionally, hydrogen from a feed stream, particularly air, comprises (a) initially removing water and carbon dioxide, (b) catalytically oxidising carbon monoxide and hydrogen to form carbon dioxide and water vapour, respectively, and (c) removing the oxidation products. The resulting gaseous stream may then be separated by cryogenic distillation. A pure nitrogen product can thus be produced. The steps (a) to (c) can be performed by passing the feed gas through a bed comprising at least one first layer of adsorbent for performing step (d), at least one second layer of oxidation catalyst for performing step (b), and at least one third layer of adsorbent for performing step (c). Two or more such beds may be sued with at least one bed being used to product the purified gas while at least one other is being regenerated. <IMAGE>

Description

The present invention relates to a process for separating air to produce nitrogen which is free from water vapor, carbon dioxide and carbon monoxide.

High purity gases such as nitrogen, in which impurities are present in amounts well below parts per million levels, are required in integrated circuit manufacturing to prevent defects in high density chips. Cryogenic distillation of air is typically used to produce highly purified nitrogen gas.

Removal of impurities from the feed air for cryogenic distillation is necessary for the production of high purity nitrogen. Impurities such as H₂O and CO₂ must be removed to prevent freeze-up in the low temperature sections of the plant. In addition, other impurities such as H₂ and CO must typically be removed from the nitrogen product.

A two-step process has been used for the removal of these contaminants from air in a nitrogen purification process. In the first step, compressed nitrogen is heated to temperatures between 150º and 250ºC and then contacted with a catalyst to oxidize CO to CO₂ and H₂ to H₂O. Noble metal catalysts, typically based on platinum, are commonly used for the oxidation step. In the second step, the oxidation products, CO₂ and H₂O, are removed from the compressed gas stream either by a Temperature swing adsorption process (see KB Wilson, AR Smith and A. Theobald, IOMA BROADCASTER, Jan-Feb 1984, pages 15-20) or by a pressure swing adsorption process (see M. Tomonura, S. Nogita, Kagaku Kogaku Ronbunshu, Vol. 13, No. 5, 1987, pages 548-553).

These processes, although effective, are disadvantageous for the commercial scale production of highly purified gases, particularly nitrogen gas, as a result of their high operating costs. The operating costs are high due to the extensive use of precious metal catalysts. In addition, separate vessels must be used for the catalytic treatment step and the adsorption step to remove the impurities. In addition, heat exchangers are required both to heat the gas as it enters the catalyst vessel and to cool the effluent therefrom, thus adding to the costs, both in terms of equipment and energy.

Ambient temperature processes for removing part per million (ppm) levels of impurities from "inert gas" streams (e.g., nitrogen or argon) are also known in the art. For example, US-A-4,579,723 discloses passing an inert gas stream containing nitrogen or argon through a catalytic bed comprising a mixture of chromium and platinum supported on gamma alumina and a second bed composed of a mixture of various metals supported on gamma alumina. The beds effectively convert carbon monoxide to carbon dioxide and hydrogen to water vapor and adsorb the resulting impurities to produce a product stream containing total impurities of less than one part per million. In addition, US-A-4,713,224 discloses a one-step process for purifying gases containing small amounts of CO, CO2, O2, H₂ and H₂O in which the gas stream is passed through a particulate material comprising nickel having a high surface area.

Processes for the ambient temperature conversion of carbon monoxide to carbon dioxide have also been described, for example, in US-A-3,672,824 and US-A-3,758,666.

However, none of these processes provide a method in which a gaseous feed air stream containing up to significant amounts of impurities can be purified by removing water vapor, carbon dioxide and carbon monoxide to obtain a highly purified air which is subsequently separated to produce high purity nitrogen.

According to the invention there is provided a method for separating air, comprising

(i) a gaseous feed air stream is purified by substantially removing water vapor, carbon monoxide and carbon dioxide contaminants by a process comprising the steps of:

(a) water vapour is removed from the gaseous feed air stream;

(b) the feed air stream from step (a) is contacted with one or more oxidation catalysts to thereby convert carbon monoxide to carbon dioxide; and

(c) carbon dioxide and, if present, water vapour are removed from the gaseous stream obtained from step (b) to obtain the purified air; and

(ii) the purified air is distilled to produce nitrogen.

If desired, hydrogen contamination can additionally be removed by catalytic reaction with oxygen in step (b) of the process according to the invention.

The resulting purified air to be distilled is substantially free of the foregoing contaminants, generally containing not more than about 1 ppm of carbon dioxide and a total of the other contaminants not exceeding about 1 ppm.

The gaseous air feed is preferably treated in a single treatment zone, preferably contained in a single vessel comprising a first adsorption section, a catalysis section and a second adsorption section. The first adsorption section contains one or more beds of a water-removing adsorbent such as activated alumina, silica gel, zeolite or combinations thereof. The catalysis section preferably contains (relatively inexpensive) catalysts for the catalytic conversion of carbon monoxide to carbon dioxide and hydrogen to water vapor. The former catalyst is preferably a mixture of manganese and copper oxides, while the latter catalyst is preferably supported palladium. If the amount of hydrogen in the gaseous air feed is such that its continued presence is not detrimental to the intended use of the purified product, then the catalyst for converting hydrogen to water vapor can be omitted. On the other hand, the production of highly purified nitrogen used in the manufacture of very high density computer chips will require the removal of even the small amount of hydrogen typically present in air. In accordance with the present invention, such amounts of hydrogen removed by converting hydrogen to water vapor using supported palladium or other suitable oxidation catalyst in the catalysis section.

The process according to the present invention can be carried out either batchwise or continuously. In either case, the treatment zone containing the two adsorption sections and the catalysis section must be periodically regenerated by purging or cleaning out the accumulated adsorbed contaminants. In a batch system, the purification of the air feed must be stopped during regeneration of the treatment section. In the continuous system, a plurality of treatment zones are used, with at least one treatment zone producing purified air while at least one other treatment zone undergoes regeneration.

In accordance with the present invention, the treatment zones can be regenerated by a purge gas near feed temperatures or at elevated temperatures in a temperature swing mode. The resulting purified air contains no more than about 1 ppm carbon dioxide and no more than 1 ppm total of other contaminants. Further specific embodiments of the invention are set out in the dependent claims.

The present invention will now be described, for example, with reference to the accompanying drawings, in which like reference numerals indicate like features, and in which:

Figure 1 is a schematic view of an apparatus for carrying out steps (a) to (c) according to the invention, which comprises a continuous shows a method for producing highly purified air using a pressure swing mode of operation;

Figure 2 is a schematic view of another apparatus for carrying out steps (a) to (c) according to the invention, in which a temperature swing mode of operation is used ; and

Figure 3 is a schematic view of an apparatus for carrying out the process according to the invention including the apparatus shown in Figure 2 in connection with an apparatus for separating the resulting purified air in a cryogenic distillation system.

The process according to the present invention typically produces highly purified nitrogen with respect to the impurities carbon monoxide, carbon dioxide and water vapor. It is to be understood that the term "impurities" as used herein refers to these gases and does not include other gases such as trace amounts of hydrocarbons which may be present in a feed air mixture and which may otherwise be considered impurities.

It is further understood that the feed air mixture contains at least carbon monoxide. Carbon dioxide and water vapor may also be present in the initial feed stream and may be generated during the oxidation step of the process.

Referring to the drawings and in particular to Fig. 1 there is shown an apparatus for operating a continuous process for the production of purified air in which a treatment zone undergoing regeneration is purified with a gas at or near the temperature of the air feed.

A feed air stream, such as atmospheric air, is fed via line 2 to a compressor 4, where the air is compressed to about 0.62 to 1.13 MPa (75 to 150 psig). The compressed air stream then passes via line 6 to a heat exchanger 8, where it is cooled upstream of its introduction via line 10 to a water separator 12 to remove liquid water therefrom. The effluent from the water separator 12 is at a temperature of about 5°C to 50°C, preferably about 20°C to 45°C.

The air gas stream then flows via a line 14, a valve 16 and a line 20 to a treatment zone 29 within a single vessel 30 which contains a lower adsorption section 34, a catalysis section 38 and an upper adsorption section 42.

The lower adsorption section 34 contains at least one bed (or layer) of a water-removing adsorbent material such as activated alumina, silica gel, zeolite, or combinations thereof, which removes water vapor and some of the carbon dioxide present in the compressed air stream. Most of the water vapor remaining in the compressed air is removed in the lower adsorbent section 34 to prevent deactivation of the oxidation catalysts present in the catalysis section 38 and to permit low temperature processing of the feed. Preferably, the adsorption section 34 comprises a predominant layer of activated alumina or Silica gel and a layer of zeolite, for example zeolite 13X or 5A, manufactured by UOP, Inc.

The air stream then enters the catalysis section 38 within the treatment vessel 30. The physical separation of the three sections of the treatment vessel 30 is accomplished by means well known in the art. For example, the adsorption section 34 and the catalysis section 38 may be separated by a stainless steel grid. A similar grid may be used to separate the catalysis section 38 from the upper adsorption section 42.

The catalysis section 38 contains a catalyst for the oxidation of carbon monoxide to carbon dioxide and a catalyst for the conversion of hydrogen to water vapor. The catalyst used to eliminate carbon monoxide is preferably a metal oxide such as nickel oxide or a mixture of oxides of manganese and copper. The catalyst used in the oxidation of hydrogen to water vapor is preferably supported palladium or other noble metal catalyst known in the art. Preferably, the air stream contacts the carbon monoxide oxidation catalyst upstream of the hydrogen oxidation catalyst.

The air thus treated enters the upper adsorption section 42 for removal of carbon dioxide and water vapor. The adsorbent that can be used in the upper adsorption section 42 preferably comprises a zeolite, activated alumina, silica gel, and combinations thereof.

The purified air thus treated, which typically contains no more than about 1.0 ppm carbon dioxide and no more than about one ppm in total of other impurities is withdrawn from the treatment vessel 30 and flows through a line 46 and a valve 58 to a line 62 where it is sent for storage or for further processing such as by cryogenic distillation.

At the same time that treatment zone 29 is purifying the air feed, treatment zone 31 is undergoing regeneration to remove accumulated contaminants. Treatment vessel 32 is essentially the same as treatment vessel 30 and contains a corresponding lower adsorption section 36, catalysis section 40 and upper section 44. The structures of the three sections 36, 40, 44 and the materials contained therein are the same as described above for sections 34, 38, 42, respectively.

After purifying the feed air for a period of time, each of the lower and upper adsorption sections 36 and 34 becomes contaminated with carbon dioxide and water, and the catalysis section 40 accumulates small amounts of carbon monoxide and hydrogen. These contaminants are removed by purging the vessel 32 with a gas which does not adversely affect the catalyst and is free of the contaminants to be removed from the vessel 32. For example, if the feed gas is air, a suitable purge gas may be highly purified nitrogen, oxygen, or mixtures thereof.

Prior to introducing the purge gas, the vessel 32 is vented to reduce the pressure therein to near atmospheric. This is accomplished by opening valve 26, thereby venting gas through a line 22 and an outlet line 28. Continuing to refer to Fig. 1, a purge gas supplied from an independent source (not shown) as a side stream from line 62 (which communicates with line 46), or as an exhaust gas from a cryogenic distillation column (not shown) at a pressure of about 107-135 kPa (1-5 psig) via line 70 to an optional compressor 68 or blower 68, respectively, where the pressure can be increased if necessary. The gas flows via line 66 to line 54 and through valve 52 into line 48 where the purge gas enters vessel 32.

The purge gas leaves the vessel 32 through a line 22 and flows through a valve 26 and is then discharged through a line 28. During this operation, the valve 24, which is connected to the line 20, is closed. After purging, it is preferred to gradually build up the pressure in the vessel 32 with purified air, referred to in Table 1 as product backfill. This repressurization can be accomplished by diverting a small portion of the purified air from the line 46 through a valve 56 and into the vessel 32 via the line 48.

Once purging and repressurization are completed, vessel 32 is again ready to begin a purification cycle. This is accomplished by closing valve 16 and opening valve 18 so that the air stream flows from line 14 to line 22 and into the lower adsorption section 36 of vessel 32. The purified air obtained from vessels 32 passes through line 48 and valve 60 into outlet line 62. At the same time, closing valve 16 will terminate the flow of air feed stream to vessel 30. Regeneration is begun by first draining vessel 30 and then introducing purge gas into line 46 via lines 66, 54 and valve 50.

The time to complete a cycle in the pressure swing mode of operation is typically from about 6 to 40 minutes. The cycle of the two-vessel process described in Figure 1 is shown in Table I. TABLE I Step Typical Valves Open Time (sec) a. Purify using vessel 30, vent vessel 32 to atmosphere b. Purify using vessel 30, regenerate vessel 32 with contaminant-free gas c. Purify using vessel 30, refill vessel 32 with vessel 30 product d. Purify using vessel 32, vent vessel 30 to atmosphere e. Purify using vessel 32, regenerate vessel 30 with contaminant-free gas f. Purify using vessel 32, refill vessel 30 with vessel 32 Total time: 20 minutes

The method according to the present invention can also be carried out by heating the purge gas to temperatures well above the temperature of the feed stream. In this temperature swing mode of operation, the temperature of the feed air stream is generally cooled below the temperature of the feed air stream used in the pressure swing embodiment, preferably in the range of about 5° to 20°C.

Referring to Fig. 2, the pressure swing mode of operation begins by pressurizing the feed air stream in compressor 4 to a pressure of about 0.62 to 1.13 MPa (75 to 150 psig). The compressed air stream is then passed to a heat exchanger 8 through line 6 and then to the water separator 12 through line 10. The compressed air exiting the water separator 12 is preferably at a temperature of about 5° to 20°C. The operation of the cleaning treatment zones 29 and 31 is the same as that described in connection with the apparatus shown in Fig. 1.

In the pressure swing mode of operation shown in Fig. 2, the vessels 30 or 32 are normally slowly pressurized using feed air and the product recirculation step as described with reference to that in the pressure swing mode of operation. For repressurization of vessel 30, valve 16 will be open, while valve 18 is used for repressurization of vessel 32. After repressurization of vessel 30, purification of the feed air takes place and the highly purified product air is sent via line 62 for downstream processing in a cryogenic distillation system. During purification in vessel 30, vessel 32 is first vented through valve 24 and line 28 and then regenerated using contaminant-free purge gas which is passed to an optional blower 68 and then to a heater 64 via line 66.

The temperature of the purge gas entering the system through line 70 is generally close to that of the feed air. Therefore, the purge gas is heated in the heater 64, preferably to a temperature of about 80° to 250°C. The heated regeneration gas passes through line 54, open valve 52 and line 48 to vessel 32 and then to line 28 via open valve 26 to thereby remove previously adsorbed contaminants.

The heat supplied to the vessel 32 from the purge gas is sufficient to desorb the contaminants contained therein. Accordingly, it is preferable to turn off the heater 64 after sufficient heat has been introduced into the vessel 32. The amount of heat required for a given vessel can be routinely determined. The flow of purge gas continues after the heater 64 is turned off to remove desorbed contaminants and begin to cool the vessel 32 in preparation for the next purification step. After the vessel 32 is sufficiently cooled, it is slowly repressurized using a portion of the feed air through the open valve 18 and line 22. The vessel 30 continues to purify the feed air during this time. After repressurization of the vessel 32, the vessel 30 undergoes the steps of draining, heating with purge gas and cooling with purge gas as described for the vessel 32. Simultaneously, the vessel 32 purifies feed air. The process can run continuously in this manner.

The complete cycle time for the temperature swing process described in Figure 2 is typically from about 8 to 24 hours, much longer than the cycle times for the pressure swing mode of operation. A complete cycle for a two-bed process operated in the pressure swing mode is given in Table 2 below. TABLE II Step Typical Valves Open Time (hours) a. Pressurize vessel 30 with feed, purify using vessel 32 b. Purify using vessel 30, vent vessel 32 to atmosphere c. Purify using vessel 30, regenerate vessel 32 with hot purge gas d. Purify using vessel 30, cool vessel 32 with purge gas e. Purify using from vessel 30, pressurize vessel 32 with feed f. Purify using vessel 32, vent vessel 30 to atmosphere g. Purify using vessel 32, regenerate vessel 30 with hot purge gas h. Purify using vessel 32, cool vessel 30 with purge gas

As previously described in connection with Figure 1, the purge gas should be substantially free of the contaminants to be removed from the system (i.e., substantially free of carbon monoxide, carbon dioxide, hydrogen, and water vapor) and should not adversely affect the components of the three sections of the vessel. If the purified air exiting line 62 is sent to a cryogenic distillation system for further processing, the exhaust gas exiting the cryogenic system can be advantageously used as the purge gas.

A system for transferring the purified air to a cryogenic distillation system is shown in Fig. 3. The purified air stream exiting at line 62 is cooled in a heat exchanger 76 against the returning product streams 78 and 86 and the purge gas stream 80. The warmed product streams 72 and 74 are taken as products and sent to user equipment or for storage. The warm purge gas stream 70 is used to regenerate the purification vessels as previously mentioned. The cold purified air stream 82 exiting the exchanger 76 is further cooled in a turboexpander 84 to produce a stream 88 which is distilled at cryogenic temperatures in a column 90 to produce two product streams 86 and 78 and an off-gas stream 80 which is used as the purge gas. The cryogenic distillation system is typically of a conventional type, the operation and construction of which is well known in the gas separation art.

The process according to the invention is further illustrated by the following examples.

EXAMPLE I

A single vessel process was operated in a pressure swing mode. The vessel contained an initial layer of 12.26 kg (27 lbs) of a commercially available activated alumina, then a layer of 0.77 kg (1.7 lbs) of 6 x 14 mesh hopcalite (a mixture of manganese and copper oxides manufactured by MSA of Pittsburgh, Pennsylvania) and a final layer of 5.27 kg (11.6 lbs) of the activated alumina. Water saturated feed air at a temperature of 25°C, a pressure of 1.07 MPa (140 psig), a flow rate of 0.65 m3/min (23.0 standard cubic feet per minute) and containing approximately 350 ppm CO2 was passed through the vessel using the cycle sequence shown in Table I. Carbon monoxide at a controlled flow rate was mixed into the feed air to give a feed gas stream carbon monoxide concentration of 5.5 ppm. The vessel was regenerated at the feed temperature (25ºC) using contaminant-free nitrogen at an average flow rate (averaged over the entire cycle) of 16.6 m³/min (9.7 standard cubic feet per second).

The gas leaving the vessel contained less than 0.1 ppm H20, 1 ppm carbon dioxide and no carbon monoxide. The carbon monoxide in the feed air was converted to carbon dioxide by the hopcalite layer and the remaining carbon dioxide and oxidizers were removed by the second activated alumina layer.

EXAMPLE II

A single vessel similar to that in Example I was charged with a first layer of 12.26 kg (27 lbs) of commercially available activated alumina, a second layer of 0.77 kg (1.7 lbs) of 6 x 14 mesh hopcalite, a third layer of 0.68 kg (1.5 lbs) of a catalyst containing 0.5 wt.% palladium supported on alumina (manufactured by Engelhard Corporation), and a final layer of 4.54 kg (10 lbs) of activated alumina. The process was operated under the same conditions of Example 1 with the feed air containing 5.5 ppm of added CO and 2.0 ppm of added hydrogen in addition to the amounts of H₂O and CO₂ expressed in Example 1. The gas leaving the vessel contained no H2, no CO, less than 0.1 ppm H2O, and less than 1 ppm carbon dioxide.

EXAMPLES III-VI

The process of the present invention was carried out in accordance with the scheme shown and described in Figure 2, wherein the purge gas was heated to a temperature exceeding the temperature of the feed gas air.

The lower adsorption section 34 of the vessel 30 was coated with a layer of 5.23 kg (11.5 lbs) of activated alumina, having thereon a layer of zeolite 13X in an amount of 2.59 kg (5.7 lbs). The catalysis section 38 was loaded with 0.32 mm (1/8") Carulite 200 pellets (a mixture of manganese and copper oxides manufactured by Carus Chemical Company of Ottawa, Illinois) in the amounts shown in TABLE IV. The upper adsorption section 42 was provided with 2.45 kg (5.4 lbs) of zeolite 13X.

A water-saturated air stream pressurized to 0.65 MPa (80 psig) and containing approximately 350 ppm of carbon dioxide and varying amounts of carbon monoxide was forwarded to vessel 30 at the rate of 0.82 m³/min (28.8 standard cubic feet per minute). Simultaneously, a regeneration flow of purge gas at the rate of 0.14 m³/min (5.1 standard cubic feet per minute) was used to remove contaminants from vessel 32. At a feed temperature of 4.4ºC, a regeneration gas temperature of 121ºC was used. Using a higher feed temperature of 12.5ºC, a regeneration gas temperature of 148.9ºC was used.

The time required to complete each step of the purification and regeneration cycles is shown in TABLE III. TABLE III Step Time (hours) Vessel pressurization Food purification Vessel draining Heating with contaminant-free nitrogen Cooling with contaminant-free nitrogen Total 12.0 hours

As seen from TABLE III, the time used to complete a single cycle of purification and regeneration is 12.0 hours during the temperature swing adsorption mode, or approximately twenty times longer than the time required to complete a single cycle in the pressure swing adsorption mode of operation.

TABLE IV shows that in each of Examples III-VI the conversion of carbon monoxide to carbon dioxide is very high. This is achieved at very low feed temperatures in accordance with the present invention in part because substantially all of the water vapor has been removed. TABLE IV Example Number Feed Temperature (ºC) ((ºF)) Feed CO Conc. (ppm) Amount of Carulite (kg) (lbs) Average Conversion of CO to CO₂ (%)

In all examples, the gas leaving the lower adsorption section contained less than 0.1 ppm of water vapor, and the gas recovered from the upper adsorption section contained less than 1 ppm of carbon dioxide.

Claims (14)

1. A method for separating air comprising (i) a gaseous feed air stream is purified by substantially removing water vapor, carbon monoxide and carbon dioxide contaminants by a process comprising the steps of: (a) water vapour is removed from the gaseous feed air stream; (b) the feed stream from step (a) is contacted with one or more oxidation catalysts to thereby convert carbon monoxide to carbon dioxide; and (c) carbon dioxide and, if present, water vapour is removed from the gaseous stream obtained from step (b) to obtain the purified air; and (ii) the purified air is distilled to produce nitrogen. 2. A process according to claim 1, wherein step (a) comprises contacting the gaseous feed stream with a water vapor removing adsorbent. 3. A process according to claim 2, wherein the water vapor removing adsorbent is selected from the group consisting of activated alumina, silica gel, zeolites, and combinations thereof. 4. A process according to any preceding claim, wherein the oxidation catalyst in step (b) is a mixture of manganese and copper oxides. 5. A process according to any preceding claim, in which the gaseous feed air stream is also substantially freed of hydrogen, wherein in step (b) the feed from step (a) is additionally contacted with an oxidation catalyst to convert hydrogen to water vapor. 6. A process according to claim 5, wherein the oxidation catalyst for converting hydrogen to water vapor is supported palladium. 7. A process according to any preceding claim, wherein the feed stream is compressed to a pressure in the range of 618 to 1135 kPa (75 to 150 psig) upstream of step (a). 8. A process according to any one of the preceding claims, in which steps (a) to (c) are carried out by passing the feed gas through a bed comprising at least a first layer of the adsorbent for carrying out step (a), at least a second layer of the oxidation catalyst for carrying out step (b) and at least a third layer of the adsorbent for carrying out step (c). 9. A process according to claim 8, wherein a plurality of said beds are used, at least one of said beds is used to remove contaminants from the gaseous feed stream, and at least one of said beds is simultaneously purged to remove contaminants present therein. contained in it. 10. A process according to claim 8, wherein the step of purging comprises reducing the pressure in the bed and passing a purge gas therethrough at a pressure below the pressure of the feed stream and at an ambient temperature, the purge gas being substantially free of the contaminants. 11. A process according to claim 8, wherein the step of purging comprises reducing the pressure in the bed and passing a suitable purge gas therethrough at a temperature in the range of from 80° to 250°C, the purge gas being substantially free of the impurities. 12. A method according to claim 11, further comprising passing an additional amount of purge gas through the treatment zone to reduce the temperature thereof. 13. A process according to any preceding claim, in which in step (a) carbon dioxide contamination is also removed from the feed stream by adsorption. 14. A process according to claim 13, wherein in both steps (a) and (c) a first adsorbent is used to remove water vapor and a second adsorbent is used to remove carbon dioxide.